As an example of an optical information recording medium, there can be mentioned a phase-change information recording medium with respect to which information can be recorded, erased, and rewritten by an optical means using a laser beam. The recording, erasing, and rewriting with respect to the phase-change information recording medium are performed by allowing the phase change material composing the recording layer thereof to change reversibly between a crystalline state and an amorphous state. Generally, the recording of information is performed by heating the recording layer to a temperature higher than its melting point by irradiation with a laser beam at a high power (recording power), melting the irradiated region, and then cooling it rapidly to form an amorphous phase. In contrast, the erasure of information is performed by heating the recording layer to a temperature higher than its crystallization temperature and lower than its melting point by irradiation with a laser beam at a power (erasing power) lower than that used for recording so as to increase the temperature of the recording layer, and then cooling it gradually to form a crystalline phase. The crystalline region and amorphous region thus formed have different reflectances from each other, and thereby information can be reproduced. As the difference between these reflectances increases, higher quality of reproduced signals can be achieved.
As an example of the phase-change information recording medium, there can be mentioned Blu-ray Disc media commercially available today. The Blu-ray Disc media can be used for digital high-definition broadcasting, and have a recording capacity of 25 GB (one layer) or 50 GB (two layers on one side) and a transfer rate of 36 Mbps (1× speed). Examples of the material used for the recording layer included in a rewritable Blu-ray Disc medium include a material having a composition located on a line extending between Ge50Te50 and Sb40Te60 (see Patent Literature 1), a material having a composition located on a line extending between Ge50Te50 and Bi40Te60 obtained by substituting Sb with Bi (see Patent Literature 2) in the composition located on the line extending between Ge50Te50 and Sb40Te60, and a material that contains Sb as its main component (near 70 atom %) and is based on a composition located in the vicinity of an eutectic point of SbTe (see Patent Literature 3).
The SbTe eutectic material is characterized in that it has a wider margin with respect to the recording linear speed. It is possible to control the crystallization ability of the recording layer by an amount of Sb contained as the main component. A larger amount of Sb can enhance the crystallization ability further, making it possible to record information at a higher linear velocity. Moreover, the above-mentioned composition located in the vicinity of the eutectic point has a large amount of optical change between the crystalline phase and amorphous phase, and thereby high signal quality can be achieved. Since addition of Ge, Ag—In or the like into SbTe, for example, can increase the crystallization temperature and enhance the signal storage stability, such ternary and quarternary materials have been developed as materials at a level of or close to practical use.
As an example of the configuration of the Blu-ray Disc media, there can be mentioned a configuration in which a reflective layer, a first dielectric layer, a first interface layer, a recording layer, a second interface layer, a second dielectric layer, and a cover layer are formed in this order on a surface of a substrate.
The first dielectric layer and the second dielectric layer have a function of enhancing the optical absorption efficiency of the recording layer by allowing their thicknesses to be adjusted, increasing the difference between the reflectance when the recording layer is in the crystalline phase and the reflectance when the recording layer is in the amorphous phase, and increasing the signal amplitude. The first dielectric layer and the second dielectric layer also have a function of protecting the recording layer from moisture, etc. As an example of the material used for these dielectric layers, a mixture of ZnS and SiO2 can be mentioned. This material is an amorphous material, and has low heat conductivity, high refractive index, and high transparency as its properties.
The first interface layer and the second interface layer are provided to prevent the elements composing the first dielectric layer and the second dielectric layer from diffusing into the recording layer when rewrite recording is performed repeatedly, and to prevent the rewriting property of the recording layer from being changed. As an example of the material used for the interface layers, there has been disclosed a material containing ZrO2 and Cr2O3, for example (see Patent Literature 4, for example). This material is excellent because it has a high transparency with respect to a blue-violet wavelength region (near 405 nm) laser and also has a high heat resistance because of a high melting point.
Optically, the reflective layer has a function of increasing an amount of light to be absorbed by the recording layer. Thermally, the reflective layer has a function of diffusing promptly the heat generated in the recording layer and cooling the recording layer rapidly so that the recording layer becomes amorphous easily. Furthermore, the reflective layer also has a function of protecting the recording layer, the interface layers, and the dielectric layers from the environment in which the medium is used. Therefore, as the material for the reflective layer, an Ag alloy with a high heat conductivity has been used preferably.
Further increases in the capacity and transfer rate will be required in the future, and various techniques have been studied accordingly. As one of the techniques for increasing the capacity, it is considered providing two or more information layers on one side. When this technique is used, the information layer disposed on the laser beam incident side is required to have a high transmittance that allows the laser beam to pass therethrough and the information layer disposed far from the laser beam incident side is required to have a high reflectance because the laser beam that is incident from one side of the information recording medium is used to reproduce the change in reflectance of each recording layer. Thus, in the information layer on the laser beam incident side, it is necessary to reduce the thickness of the recording layer to increase the transmittance, and in the information layer disposed far from the laser beam incident side, it is necessary to increase the thicknesses of the recording layer and the reflective layer to increase the reflectance.
As a technique for increasing the capacity further, it also can be considered increasing the recording (mark/space) density in a plane to increase the recording capacity per information layer from 25 GB to 30 GB or 33.4 GB, for example. Since an increase in the recording density shortens the interval between a recorded mark (=an amorphous region) and a pace (=a crystalline region), the material composition of each of the layers and the configuration of the information recording medium need to be made suitable for high density recording.
Furthermore, since the resultant higher transfer rate shortens the period of time during which the recording layer is irradiated with the laser beam, it is necessary to record and erase information in a shorter time. This requires to use a phase change material in which atoms can move faster, having an enhanced crystallization rate.